Self-defense can be a time consuming process. Most individuals do not have time patrol their neighborhood, look out the windows every couple of minutes, or stare at boring video feeds at all hours of the day or night. It makes sense to take advantage of Division of Labor in this case.
However, outsourcing self-defense services is not without its’ challenges. One question is how do you prevent the service provider from harming you with the power they possess? This service provider knows your identity, location, schedule, and so on. This places them in a considerably powerful position. Of course, abuse of that power means that they lose a customer, and they may sour their reputation. Assuming that you are wealthy an individual security guard might weigh the risk / reward of stealing from you while you are away.
The other danger is that self-defense providers may compete with each other aggressively for customers. What mechanisms could be utilized to prevent this? Another issue is that the service provider might provide your information to one of their criminal contacts in exchange for some of the action.
If you are paranoid about your service provider you could potentially enter into a contract with other self-defense providers as well. They could act as a check against each other, and ensure that more than one set of eyes are monitoring your property at all times. The end result might be that you are paying three different self-defense providers. This would imply that they would all need to collude with each other to intentionally harm you.